When I got it, two oil bath resistors were *way* off value (202 and 100 ohms
low respectively), and the S2 shunt resistors were about 1.3 ohms high which
was far enough off to prevent S2 calibration. There were were also sundry
other problems like two open circuits in the final decade, a badly worn trim
pot, and sundry wires broken at solder joints.
The resistors in a 720A -- even the pad resistors -- do not drift that
much by themselves under normal use. Trim pots do not get used enough
to wear out under normal operation. If you have some that are worn, the
instrument has been abused. This is further indicated by the broken
wires. I have *never* seen a broken wire in a 720A.
Unfortunately, it seems clear that your 720A was traumatized in its
earlier life, and some of the divider resistors were damaged (in
addition to other probable damage, based on your reports). There really
isn't anything you can do but replace the bad resistors with equal or
better parts. Trimming them as you have done is just a temporary
band-aid. Resistors that have been traumatized will never allow the
instrument to meet its time and temperature drift specifications, and
you will be going back inside to replace (and re-replace) pad resistors
every time you want to use it.
Worse, the problem goes deeper than that. Even replacing the resistors
you know are bad will not restore the instrument to its accuracy and
stability specs. The techniques you are using to identify the bad
resistors will only identify ones that have been grossly damaged -- they
will not identify all of the resistors that have been damaged and are
preventing the instrument from meeting its specifications.
Further, the very act of replacing resistors in a 720A is likely to
cause additional damage -- to the resistors you are soldering, the
resistors attached to them, and the infrastructure of the instrument.
Add to this the fact that only a few qualified replacement resistors
will cost more than a properly-working used 720A, and you can see that
repairing a traumatized 720A -- if you can do it at all -- is not
cost-effective for an amateur.
The only 720As I've ever seen that worked correctly after being
traumatized had been repaired by Fluke at astronomical cost. I hate to
say it, but if you expect to use a 720A to calibrate meters of 6-1/2
digits and up, you will eventually learn that you need to replace the
one you have.
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